The spinal column is a flexible column formed from a linear series of vertebral bones separated by intervertebral discs. These discs reduce friction between adjacent vertebrae and absorb compression forces applied to the spinal column. A vertebra includes an anterior body and a posterior arch that surrounds the spinal cord. Spinal nerves extend from each side of the spinal cord and exit the column at the vertebral foramen, which is formed by the posterior arch. Articular processes, including the superior articular process and the inferior articular process, are small flat projections on the surfaces of the arches.
There are four facet joints associated with each vertebrae, and these joints interlock with adjacent vertebrae. In this manner, facets on the opposing processes determine the range and direction of movement between adjacent vertebrae, hence the flexibility of the spinal column. The facet joints maintain spinal stability, protect the disc from excessive stress, and assist the discs in allowing motion and controlling shear forces. These joints are vulnerable to degenerative spinal disorders.
Degenerative disc disease is typically caused by a loss of disc space height, leading to a narrowing of the neural foramen and subsequent neural compression, and causing back and radicular pain. Instability of the posterior elements can lead to a condition known as spondylolisthesis, in which a vertebral body slips forward in relation to an adjacent vertebrae. This movement of the vertebral body narrows the foramen and results in painful pressure on the nerve roots.
Degenerative disc disease may often be resolved through a spinal fusion procedure using an interbody implant (one which is implanted between the bodies of two adjacent vertebrae). Such interbody implants may be formed from titanium, carbon fiber, allograft, or other suitable material including, but not limited to, biocompatible materials such as PEEK™, available from Invibio®. Implantation of a substitute graft is designed to reestablish normal disc height, provide immediate stability to the motion segment, and provide a matrix for fusion. When the implant grows into the existing bone, the fusion becomes solid and movement is eliminated at that level. A fusion procedure may also involve the surgical implantation of hardware, such as plates, screws or cages.
In order to fuse and thereby stabilize the motion segment, the disc space must be prepared prior to insertion of the interbody device. Soft tissue, such as disc material and cartilage, and other such tissue, is cleaned off the vertebral endplates so that intimate bony contact is obtained between the graft, implant and host tissue. The preparation of the disc space can be achieved with scrapers, curettes, rongeurs, drills, rasps and/or chisels. In preparing the disc space, it is important not to remove too much of the endplate in order to maintain structural integrity so that the interbody implant does not telescope into the vertebral body when normal axial loads are applied.
Posterior Lumbar Interbody Fusion (PLIF) is one surgical fusion technique used to treat degenerative lumbar disc disease. Proper distraction during a PLIF procedure must be achieved in order to gain compression of the implant. Proper distraction allows natural compression across the disc space via the annulus and other posterior elements. This compression delivered to the implant helps stabilize the implant, which avoids expulsion, and keeps the grafting material under stress, thus promoting faster fusion and bone healing.
Transforaminal Lumbar Interbody Fusion (TLIF), also referred to as an extended PLIF, was developed in response to problems associated with PLIF procedures. In the TLIF approach, the disc space is expanded by removing one entire facet joint, while a PLIF is usually performed on both sides of the facet, removing a portion of each of the joints. Removal of the entire facet joint improves visualization into the disc space, allowing removal of more disc material and insertion of a larger implant. Other procedures have been developed to provide anterior column support as well, including the Anterior Lumbar Interbody Fusion (ALIF) and extreme lateral interbody fusion techniques that access the vertebrae through the psoas muscle.
The instruments used in current procedures include design limitations that fail to address the challenges of the neural anatomy or require numerous instruments and steps that add significant time to the surgical procedure. Delivery of an interbody device requires a large amount of bone resection and neural retraction. Removal of the lamina and facet joint, which may be necessary in order to insert the implant, can potentially destabilize the motion segment. In addition, there is increased surgical time due to the more extensive bone removal and disc preparation. Destabilization of the motion segment can interfere with compression of the interbody device, especially in a “stand alone” situation in which additional hardware is not utilized. Therefore, it is necessary to balance the need to deliver an appropriately sized interbody device (to restore the appropriate disc height) without destabilizing the segment (so the necessary compression can occur).
Furthermore, existing methods of introducing interbody implants into the disc space, the freehand method and the controlled method, include disadvantages. First, existing surgical techniques in which an implant is inserted into the disc space freehand (without controlled access) and impacted into proper position present dangers to delicate neural structures. Each time an instrument is introduced or removed from the surgical site, there is a chance that delicate structures, such as the spinal cord or nerve roots, could be compromised, potentially causing severe damage. Additionally, maintaining constant distraction is a challenge, as instruments are passed by the neural structures “freehand.” Moreover, if a distractor is placed in the contralateral side of implantation, it does not always address the distraction needs of the operative side.
In existing “controlled” procedures, the instruments used to provide protected access to the disc space often occupy an excessive amount of the disc space. As a result, an implant smaller than that which the disc space is capable of accepting must be used. This smaller implant does not restore proper disc space height and therefore the stability of the fusion is compromised. Furthermore, these instruments require much more bony resection in order to be placed correctly, thus further destabilizing the spine.
In another existing controlled procedure, a series of instruments slide through a channel of a wide profile distractor. This design limits visualization and neural retraction. In addition, the design introduces a significant amount of lateralization to the placement of the implants for PLIF procedures: the implants are spaced farther apart from one another and must be implanted with a significant amount of space between them. Further, this delivery design has poor anatomical fit, as it creates a dead space between the external wall of the retractor and the load bearing surface of the implant.
Thus, there is a need for a system that allows delivery of the appropriate size interbody device (in order to restore disc height), and adequate neural retraction without compromising visualization of the surgical site.